A member with a surface to which hydrophilicity is added is used in various areas such as anti-fogging, dew condensation prevention, stain-proofing, printing of a water based coating, and the like. Recently, a hydrophilic member (member with a surface to which hydrophilicity is added) using a titanium oxide based photocatalyst film is developed and applied to an exterior wall, a window glass, a rearview mirror of an automobile, and the like. A titanium oxide film changes in surface state when it is irradiated with ultraviolet rays included in the sunlight and becomes hydrophilic, and simultaneously an organic matter adhering to the surface is oxidized and decomposed by photocatalysis, thereby exhibiting high hydrophilicity. On a building material or a window glass to which the titanium oxide based photocatalyst film is applied, stain adhering to its surface is washed away by rains, thereby obtaining a stain-proofing effect.
When the member which becomes hydrophilic by light is actually applied, a state that there is no irradiation with light becomes a problem. The titanium oxide film decreases in hydrophilicity in a relatively short time, and thus when it is applied to, for example, an anti-fogging agent for a rear view mirror of an automobile, the anti-fogging effect is insufficient during night or in a state of being stored in a garage. In some of such members, it is devised to elongate the duration of the hydrophilicity by mixing a silicon dioxide, which is a hydrophilic oxide, or the like with the titanium oxide, but sufficient performance has not obtained. Moreover, ultraviolet rays are necessary as excitation light, and thus excitation light may become insufficient in the shade or indoors. For supplementing insufficient excitation light, nitrogen or sulfur is added to the titanium oxide, or carrying by platinum is performed. However, the wavelength range of available light does not enlarge so much, and performance applicable to indoor use has not been obtained. The duration of hydrophilic effect is similar to that of conventional titanium oxide, and the hydrophilicity decreases in a short period in a dark place.
Tungsten oxide is widely used as a dielectric material for electronic devices, an optical element material, an electrochromic material, and a gas sensor material, and is further known as a visible light responsive photocatalyst material. The band gap of the tungsten oxide is 2.5 eV, and visible light in the vicinity of 450 nm can be used as excitation light, whereas ultraviolet rays of only 380 nm or lower can be used for the titanium oxide. Accordingly, the tungsten oxide can be used as a photocatalyst by the wavelength range of light from a fluorescent lamp or light bulb. The tungsten oxide is also known to exhibit hydrophilicity by light irradiation, and mainly there are reported films produced by a vacuum deposition method, a sputtering method, a laser ablation method, a sol gel method, and the like.
JP-A 2001-152130 (KOKAI) describes a photocatalyst material obtained by sputter-depositing tungsten oxide on a substrate, where tungsten oxide having a triclinic crystal structure is mainly used. JP-A 2001-152130 (KOKAI) discloses obtainment of hydrophilicity by exciting a tungsten oxide film with visible light. Specifically, it describes that a contact angle (initial value) of a sputter-deposited tungsten oxide film with water is in the range of 10° to 30°, and the contact angle with water becomes 5° or less when the tungsten oxide film is irradiated with ultraviolet rays and about 20 minutes elapse thereafter. J. Phys. D: Appl. Phys. 40 (2007) 1134 discloses that a tungsten oxide film which is formed by a thermal deposition method or sol-gel method and heat treated thereafter at 400° C. exhibits hydrophilicity.
A conventional tungsten oxide film exhibits hydrophilicity upon excitation by light, and performance in a state that light is insufficient becomes a problem. When hydrophilicity is made by applying heating, heat resistance of the substrate becomes a problem, and for a member with a large area, the method of heating also becomes a problem. Further, when hydrophilicity is made in post-processing such as light irradiation or heating, its duration is short and regular light irradiation or heating becomes necessary in a short period. Moreover, it is not possible to remove organic matters by only the hydrophilicity, and thus when an organic matter such as an oil component adheres to the surface, it must be removed by sufficient rainwater, water washing, or the like. This poses a problem of limitation on use environment. Accordingly, the performance to oxidize and decompose organic matters on the surface by photocatalysis is also needed, but sufficient photocatalytic performance has not been obtained with conventional tungsten oxide films.
In order to form a uniform film using a tungsten oxide powder, a fine powder is needed. As a method of producing the tungsten oxide powder, there is known a method (see JP-A2002-293544 (KOKAI)) to heat an ammonium paratungstate (APT) in the air to obtain a tungsten trioxide powder. By the method to heat the APT in the air, the tungsten trioxide powder with a primary particle diameter of 0.01 μm (BET specific surface area=82 m2/g) is obtained. As a method of obtaining a fine powder of tungsten oxide efficiently, thermal plasma processing is described in JP-A2006-102737 (KOKAI). By applying the thermal plasma processing, a fine powder with a particle diameter of 1 nm to 200 nm is obtained. However, when the tungsten oxide powder produced by applying one of these methods is used as it is, the hydrophilicity by light is insufficient, and the hydrophilicity cannot be sustained for a long period. Thus, in the current situation, a tungsten oxide film exhibiting practical hydrophilicity has not been obtained.